1. Field of the Invention
The present invention relates to liquid crystal image bars for electrophotographic printers and, more particularly, to transmissive nematic image bars in which all electrodes are normally energized for nontransmission of light therethrough and which selectively pass bursts of light for erasure of precharged regions on a moving photoconductive member of the printer during the transient state. This transient state is caused by the interruption and reapplication of a voltage to selected electrodes.
2. Description of the Prior Art
This invention is an improvement over U.S. Pat. No. 4,595,259 to Perregaux, granted June 17, 1986. In this patent, crossed polarizers are used on either side of a nematic liquid crystal device to prevent the passage of light in the electrically driven state. The liquid crystal device is configured as a single array of dot shutters and is driven by a single frequency voltage source. A nematic liquid crystal material having a positive dielectric anisotropy is used. The electrically driven stable state of the liquid crystal material would permit the transmission of light except for the crossed polarizers. When selective ones of the dot shutters are to transmit light, the electric field across the electrodes forming the dot shutters is interrupted. Within one millisecond after field interruption, about 90 percent of the transmissable light passes through the selected dot shutters. The first transmission maximum occurs and diminishes to about 90 percent of the transmissable light after two or three milliseconds, but in all cases where speed is critical, the transmission burst can be interrupted before it has reached its maximum through reapplication of the electric field and still provide enough energy to form one line of a latent electrostatic image on a moving photoconductive member. Once the field has been reapplied and the system brought back to its dark state, if a second succeeding line of latent image requires the passage of light from the same dot shutter, the voltage to the electrodes is interrupted again to pass another beam of light. The energized time period for rendering the image bar non-transmissive between any two bursts of light is so short relative to the movement of the photoconductive member that there is no undischarged space between succeeding adjacent spots or pixels. Therefore, high quality latent images are produced. Such an arrangement provides a liquid crystal dot shutter type image bar suitable for use in an electrophotographic printer, wherein the photoconductive member moves at a speed of typically two inches per second. This arrangement is relatively independent of temperature and gap thickness, so that it is very cost effective and suitable for low cost printers.
However, the image bar disclosed in U.S. Pat. No. 4,595,259 exhibits limited contrast at low voltage primarily because of the incomplete tilt of the liquid crystal molecules in their energized state mostly in the direct vicinity of the substrate surface. That is, the molecules are not perfectly parallel to the electric field or perpendicular to the glass substrates which sandwich the liquid crystal material. As a consequence, the linearly polarized light entering the device comes out of it elliptically polarized and is incompletely absorbed by the second crossed polarizer. None of the prior art discusses or solves this problem.
U.S. Pat. No. 4,497,542 to Kaye discloses the use of two parallel stacked liquid crystal cells, whereby a beam of light passing through one of the cells in a direction normal thereto passes through both cells. The pair of cells are positioned between a pair of polarizers and one is rotated 180 degrees relative to the other about an axis normal to the cells. The liquid crystal molecules of the cells tilt in opposite directions. This has the effect of compensating for a lack of beam collimation. The patent to Kaye addresses the same type of problem as the subject invention, but solves it in a totally different manner. Kaye proposes to compensate for the imperfection of one sign in one device with the imperfection of the opposite sign in a second device of the same type. No attempt is made to use standard optical retarders with which the amount of compensation can be adjusted.
U.S. Pat. No. 4,408,839 to Wiener-Avnear discloses a twisted nematic liquid crystal light valve which compensates for residual birefringence. Impinging light passes through two separate liquid crystal layers having their optical axes twisted oppositely along an axis parallel to the direction of light propagation and aligned perpendicular to one another at the interface between the two liquid crystal layers. The light beam becomes elliptically polarized as it passes through the first liquid crystal layer. The light beam polarization is then transformed from an ellipse to a straight line as the beam traverses through the second liquid crystal layer. In this patent, the imperfection of a right-handed twisted device is compensated with the imperfection of an identical device that has a left-handed twist. As in the patent to Kaye, no attempt is made to use standard optical retarders.
U.S. Pat. No. 4,212,048 to Castleberry discloses the use of a rear illumination prism containing a polarizer to improve the contrast of a dye loaded liquid crystal display device in the reflection mode. It basically suppresses the portion of the light that is polarized in the undesirable direction. In another embodiment of this invention, a prism is used only to redirect a narrow pencil of light in one specific direction without acting on its state of polarization.
U.S. Pat. No. 4,019,808 to Scheffer discloses the use of a birefringent retarder to turn a normally black and white twisted nematic display device into a two color device. In essence, the grey level contrast is replaced by a color contrast. No attempt is made to compensate for any imperfect tilt of the liquid crystal molecules inside the liquid crystal device. On the contrary, the present invention proposes to correct for this imperfect tilt, and improve the grey level contrast without converting it to a color contrast.
U.S. Pat. No. 3,912,369 to Kashnow discloses a liquid crystal display having a polarizer, a liquid crystal cell, a quarter-wave plate, and a reflector. In different embodiments, negative and positive dielectric anisotropy liquid crystal materials are utilized. The main purpose of this patent is to retain the improved contrast of wave rotation provided by a regular twisted nematic device while eliminating a polarizer by placing it between a polarizer and a quarter-wave plate, instead of placing it between two polarizers, as is used in most cases. However, no attempt is made to improve a two polarizer system with an added optical retarder.
U.S. Pat. No. 3,900,248 to Nagasaki discloses a dye loaded liquid crystal device which without polarizers would have a very low contrast of roughly two. Instead of adding a polarizer, this patent adds an optical retarder to rotate the polarization after it has gone through the liquid crystal device to improve the contrast either in the reflected beam or in the transmitted beam intended to go through a second liquid crystal device. It does not address the problem of the remanent ellipticity in a two polarizer system.
U.S. Pat. No. 3,863,246 to Trcka et al discloses a method of lighting a display from the back without the interference of direct light rays. This backlighting scheme reduces bright spot characteristics through the use of a louvered material and a prism, wherein the prism is used to direct light to a display. The main purpose of this invention is to provide the exit side of the device (viewing side) with the appearance of a diffused illumination. To the contrary, the prism of the present invention is used to direct a narrow pencil of light in one specific direction.
U.S. Pat. No. 3,784,280 to Bigelow discloses a reflective liquid crystal display with a light-dark contrasting image by employing a liquid crystal cell containing a nematic liquid crystal composition of a positive dielectric anisotropy and mutually parallel molecular alignment at opposing planar surfaces of the cell. An optical polarizer is positioned on one side of the liquid crystal cell and an optically reflecting member is positioned on the other side of the cell. Unpolarized light incident on the optical polarizer is polarized at a 45 degree angle before entering the liquid crystal cell. The thickness of the liquid crystal cell is adjusted to provide a quarter-wave length relative retardation of the polarized light entering the liquid crystal cell at a 45 degree angle. The light exiting the liquid crystal cell is circularly polarized and after reflection from the optical reflecting member, reenters the liquid crystal cell with the sense of rotation reversed by the reflecting member. The polarization of the light exiting the liquid crystal cell is crossed with respect to the polarization of the polarizer and hence there is complete extinction and a resulting dark surface appearing on the polarizer.
A light region is produced from the liquid crystal display by changing the molecular orientation of the liquid crystal composition such that the alignment of the molecules is in a direction substantially orthogonal to the opposing surfaces of the planar cell walls. Under these conditions, the polarized light exiting from the polarizer passes through the liquid crystal cells and is reflected back through the cell with no polar rotation so that a light region is produced at the viewing surface of the polarizer. By providing means to controllably alter the molecular alignment of the liquid crystal composition in selected regions, light-dark contrasting images are readily displayed at the viewing surface of the polarizer. Here, the liquid crystal device itself is used as an electrically switchable optical quarter-wave retarder. It has nothing to do with the addition of a retarder to a two polarizer system for the sole purpose of boosting the contrast.